1. Field of the Invention
The present invention relates to a vibration isolating device for use in an engine mount or the like for motor vehicles and particularly, to the device of which durability is improved in the case of employing an insulator provided with a thin wall portion.
2. Description of the Related Art
As a vibration isolating device for use in an engine mount or the like for the motor vehicles, there is known a liquid sealed vibration isolating device provided with a generally dome-shaped insulator which forms a portion of a liquid chamber, wherein a principal vibration is inputted from a vibration inputting section provided around a center portion of the insulator. Also, there is proposed that the thickness of the insulator is relatively varied to provide a thick wall portion and a thin wall portion, and the whole of the insulator is formed to be low spring constant by the thin wall portion so as to control an increase in liquid pressure when the vibration is excessively inputted (for example, see a patent reference 1).    Patent reference 1: Japanese patent laid open publication No. 2000-186739.
FIG. 7 is a schematic illustration showing an insulator 103 having such thin wall portion, in the input direction of the principal vibration. The thin wall portions 111 are formed in a cross shape, and between the neighboring thin wall portions 111 there is provided a relatively thick generally fan-shaped thick wall portion 112 (shown in cross striped hatching). At the central area there is provided a vibration input portion 104 to which the principal vibration is inputted.
By the way, the thick wall portions 112 are formed in a fan-shape by the provision of the thin wall portions 111, and the width “a” of an outer peripheral restrained portion 112A is remarkably greater than the width “b” of a central restrained portion 112B. Therefore, when the elastic deformation mainly comprised of tension and compression deformation is caused between the vicinity of the outer peripheral restrained portion 112A and the vicinity of the central restrained portion 112B of the thick wall portion 112 by the input of the vibration in the direction (hereinafter, referred to as “horizontal direction”) meeting at right angles with the input direction (hereinafter, referred to as “vertical direction”) of the principal vibration, there is a marked difference in rigidity between the vicinity of the outer peripheral restrained portion 112A and the vicinity of the central restrained portion 112B. As a result, the stress concentration is developed on the side of the central restrained portion 112B of less rigidity so that the durability may be decreased.
While there are cases where a rigid body covering portion for integrally covering the vibration input portion is provided in the central position of the thick wall portion, the thickness of this rigid body covering portion is about 1.5 mm at the thickest possible manufacturing limit. Then, since the movement is controlled by the vibration input portion of rigid body, the thick wall portion is divided by the central vibration input portion so that the free length (see “f” in FIG. 3) of elastic deformation becomes comparatively short. Accordingly, the stress concentration in the vicinity of the central restrained portion 112B is increased, whereby there has caused a disadvantage such as a crack or the like in this area by a comparatively small number of repeated vibrations. Hitherto, there has not been known that this thickness has an influence upon the durability improvement.
Further, in the case where the spring constant in the vertical direction is Z, and the spring constant in the dual directions meeting at right angles on the same horizontal plane is X and Y, when the spring constant in the horizontal direction is greater than the one in the vertical direction, the above mentioned stress concentration is developed remarkably so that in view of the durability it is not possible to increase the spring constant in the horizontal direction large enough to be expected. Accordingly the spring constant ratio of Z:X:Y in each of the directions is about 1:0.8:0.4 at the most. In recent years, however, it has been required that the insulator has the largely improved spring constant ratio such that each spring constant ratio of X and Y to Z is 1.0 or greater. Therefore, it is a principal object of the present invention to make it possible to improve the durability of the insulator even if the thin wall portion is provided. Also, it is another object to improve the spring constant ratio in such a manner as to make the spring constant in the horizontal direction greater.